Plastic elements in the form of films and preformed members are widely used in packaging as well as structural constructions. The plastic member is often conveniently attached to another member particularly another plastic member by a bond, with or without an interposed medium.
The processing and working of plastic materials and elements thus generally involve various cutting, forming, sealing and bonding functions. For example, flat plastic parts may be formed from film-like material by die cutting. Multiple plastic layers and thin plastic films may be attached by applying of heated elements. For example, sealing of plastic objects to each other, particularly with a straight line bond, is often made using a metal wire, or narrow band of material, covered by a protective plastic release layer. The wire-like element is placed in engagement with the plastic layers and an electric current is passed through the element. The current flow results in resistance heating the element and the temperature of the wire increases to a temperature operable to fuse the plastic layer or layers to form a thermal fusion bond. After the necessary time of heating, the current is removed, the joined elements allowed to cool, thereby simultaneously cooling of the plastic to complete the seal or connection. Cutting and joining of layers may also be accomplished employing a heated element. For example, United States Patent 3,441,460 discloses a system wherein the film layers are held between a pair of clamping jaws. A resistance heated knife is moved downwardly between the clamping jaws into engagement with the plastic film. The knife functions to simultaneously sever the plastic layers by melting thereof and creating an interconnecting plastic bead on the severed edges.
Another significant method of bonding plastic elements in the use of inductive heat generation in the plastic elements. For example, the present inventor is active in the development and application of a unique inductive heating method using particulate material embedded within one or more plastic elements and which responds to a high frequency magnetic field to create heat within the element as the result of hysteresis losses. The hysteresis loss method rapidly heats the plastic to the level necessary to create a firm and reliable fusion bond. Other forms of inductive heating of the plastic member have also been suggested for bonding. For example, U.S. Pat. No. 3,909,326 discloses a method of attaching a film cover to one end of a tubular member, such as an open-mouth container. In such system, the packaging or covering member includes a metallic film having a heat sealable material on one surface which is held in firm engagement with the open edge of the container. The covered container is moved past an induction heating coil which activates the metallic film through inductive effects to generate heat within the sealing member of an appropriate level to melt the heat sealable material and heat seal the member to the container. The prior art has even suggested the use of ultrasonic systems for attaching one plastic film member to another member; for example, as shown in U.S. Pat. No. 3,970,490 which issued July 20, 1976.
The above and similar teachings have been suggested but there remains a need for a reliable method of working thin plastic films and plastic members for cutting and/or sealing to another member, and eliminating the need for expensive heat generating material such as aluminum foil in the assembly.